Freezers are used for a wide variety of purposes including for storage of food products, of materials being tested and for long-term storage of animal and human tissue and fluids. The precautions taken for each type of storage vary with the criticality of the integrity of the freezer contents.
An aspect of freezer operation with which the user must sooner or later contend involves the formation of frost. Frost impairs the ability of the freezer to retain an optimum cold temperature and, with sufficient frost buildup, can prevent tight door closure. An ill-fitting door permits warm air to migrate into the cold compartment and impair the quality of the stored contents.
At or near one end of the criticality "spectrum" are food freezers, both residential and commercial. Unless special equipment configurations are employed, the usual procedure for remedying an "over-frosted" freezer is to remove the freezer contents to other cold storage, shut down the freezer and either scrape away frost or simply let it melt as the freezer warms to room temperature.
The prior art includes some of the special equipment configurations used to help avoid total freezer shutdown. For example, U.S. Pat. No. 3,858,408 (Kenyon) relates to detachable frost shields mounted tightly against the surface of the permanent liner of a food cooler, e.g., a refrigerated ice cream cabinet.
U.S. Pat. No. 4,603,558 (McAdams) relates to a chest-type food freezer and is directed to a separate, conformably-shaped container placed into the freezer. During defrosting, the container contents are removed.
But with a food freezer, the worst that can occur in the event of heavy frost buildup is that a batch of food is ruined. However, such ruined food is replaceable at fairly nominal expense.
Laboratory freezers are at the other end of the spectrum in terms of criticality of the integrity of the freezer contents. This is so because of the types of items required to be stored without significant deterioration. Consider, for example, that laboratory freezers contain specimens of human tissue and fluids which may be vital for diagnosis, for long-term patient treatment strategies or as evidence in a legal proceeding. Such specimens may be irreplaceable, either per se or as reflective of a condition of the person at the time the specimen was taken.
For some time, companies have manufactured and sold freezers for ultra-low temperature storage of laboratory specimens. Such freezers are available in chest and upright type and Thermotron Industries of Holland, Mich., is the leading manufacturer of such freezers.
Upright freezers have a single outer door covering several inner compartment doors, each of which can be separately opened without disturbing the door of any other compartment. While the inner doors are typically made of metal, the compartment liner is sheet steel and the "rim" around each compartment opening is of plastic. And until the advent of the Thermotron invention, the inner doors were permanently attached to the freezer and could not be removed except, presumably, by using tools.
When an inner compartment door is opened, ambient air (with its "entrained" water vapor) migrates toward and around such door and the compartment opening. Because the door is extremely cold and because it is made of metal, moisture tends to condense and freeze on such door, especially the door edges. Over a short time, the inevitable frost buildup makes the door difficult to close and seal properly against the compartment rim.
When that occurs, it has been necessary to either scrape or melt away the frost. This often necessitates moving the compartment contents to another freezer or to an alternate storage area cooled by, say, liquid nitrogen. The invention addresses this problem in a unique way.